Research Projects

Transition metal dichalcogenides (TMDs) are layered semiconductors which show unique optoelectronic characteristics such a strong exciton binding energy, strong carrier-carrier interactions, and spin-dependent valley polarization in monolayer. It is possible to engineer the material properties by stacking two monolayers of different TMDs on top of each other. An interesting feature of such heterostructures is the presence of interlayer excitons (ILEs) due to coupling between the individual layers. The ILEs have received significant interest from the perspective of understanding carrier dynamics, exciton diffusion, spin-valley physics. Time-domain optical experiments have been particularly useful in understanding the various dynamics. However, most of the studies have focused on incoherent population dynamics of ILEs. As a consequence, coherent light-matter interactions in these heterostructures have not been studied in much detail. We propose to study these novel materials using two-dimensional coherent spectroscopy (2DCS). 2DCS is a powerful technique that measures the third-order nonlinear four-wave mixing (FWM) signal as a function of two frequency axes. This technique has been successfully used to study frequency-resolved homogeneous excitonic response in inhomogeneously broadened semiconductor nanostructures. 2DCS is particularly sensitive to identify and measure many-body interactions and coherent coupling. We aim to measure these properties in the TMD heterostructures. An interesting feature of TMDs is the spatial variation of the optical response. It is important to understand these variations and the underlying mechanism in order to better control these variations. We propose to develop a home-built experiment to measure the spatially-resolved optical response of the heterostructures. The spatial resolution will be achieved by scanning the excitation spot on the sample using mirrors attached to a pair of galvanometers. We will use the beam-scanning method to measure both the photoluminescence and 2D spectra. The proposed project would give us a comprehensive understanding of the excitonic physics in TMD heterostructure. A particularly interesting aspect would be to explore coherent interactions of moiré excitons that are formed for small twist angle between the constituent TMD monolayers. These experiments should further our understanding of the fundamental interactions governing the coherent light-matter interactions in TMD heterostructures. This improved understanding can be leveraged to better control and tune these interactions for optoelectronic applications.

TMD heterostructures, Interlayer excitons, moire excitons, Coherent interactions, 2D coherent spectroscopy